IEEE Electrification Magazine - December 2017 - 44

Retract
Actuator

Retract
Actuator
Moment Arm

Side-Brace
Locking
Actuator

Figure 6. The typical landing-gear kinematic constraints in commercial aircraft.

44

Aircraft Braking

EMA/EHA Controllers

Historically, aircraft brakes have been actuated with conventional hydraulic power. The 787 was the first large
commercial aircraft with EMA actuated brakes. Safran,
one of the two suppliers of electric brakes for the 787, had
the following assessments (Florian 2013):
xx
The electric actuation for brakes had slightly higher
system weight than hydraulic actuation.
xx
Overall aircraft availability was superior, even with
slightly lower equipment reliability (due to an increase
in parts count). High availability means lower costs.
xx
Installation and maintenance costs for the electric actuation system were lower than the hydraulic system.
xx
The performance of the electrically actuated braking
system was identical to the conventional system.
When Boeing completed the trade study between electric and hydraulic braking, the electric system proved to be
more beneficial. As an additional benefit over the hydraulic braking system, electric braking systems do not have a
propensity to catch on fire during maximum energy
rejected takeoff (RTOs). The maximum energy RTO occurs
when the pilot decides to abort the takeoff after accelerating to just below the aircraft critical speed (and at maximum weight). Maximum braking is necessary to
decelerate the plane before running out of runway. The
RTO represents the maximum energy the brakes must
absorb. This amount of energy can increase the brake
temperature to a point that exceeds the flash point of
hydraulic fluid (Fernandes 2010).

EMAs and EHAs require electronic controllers. In servo
applications such as flight-control actuation, the controllers
receive position commands and control the motor to
achieve the commanded position from the flight-control
computer. There are three (or more in some aircrafts) flightcontrol computers that all send commands to the actuator
controller. Modern actuator controls have complex digital
implementations that receive the three commands, determine which command is valid (by voting on the three commands), close high-speed motor- and motion-control loops,
report feedback telemetry, and continue to operate during
fault conditions. Redundancy inside the control electronics
to continue operation in the event of a failure in one channel drives additional complexity. This redundancy requires
monitor systems that activate/deactivate control channels
as necessary.
With the adoption of digital controllers, rigorous standards in firmware and software development have been
mandated by the certification authorities to ensure safety.
This process rigor comes at the expense of large nonrecurring engineering (NRE) costs. This rigor also makes the
development time of controllers extend over long periods.
While the high NRE costs and long lead times are certainly
negatives in any program, the digital controller also provides significant advantages. Complex control algorithms
can be programmed easily into the digital controller that
optimizes the actuation system performance. Additionally,
digital controllers allow the development of prognostic

I E E E E l e c t r i f i cati o n M a gaz ine / DECEMBER 2017



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